The present invention generally pertains to ballast circuits for low pressure gas discharge lamps and is particularly directed to a solid state ballast circuit that enables the lamp to quickly reach steady state illumination without causing mercury migration within the lamp.
A low pressure gas discharge lamp, such as a fluorescent lamp, typically has two filament-type electrodes at opposite ends of the lamps; and the lamp is illuminated when a high AC voltage is applied across the opposite electrodes of the lamp.
A ballast circuit for such a lamp is designed to provide a high voltage to the lamp upon turn-on to ignite the lamp, and then a constant current at a lower voltage for enabling uniform steady-state illumination. A basic prior art ballast circuit includes a loosely coupled transformer with the secondary of the transformer being connected across the opposite electrodes of the lamp. As the gas mixture within the lamp heats up, the internal resistance of the lamp between the opposite electrodes decreases. But, as the lamp draws more current, the core of the transformer becomes saturated and the voltage across its secondary winding decreases, so that following ignition of the lamp, the current supplied to the lamp decreases to a constant level for providing steady state illumination.
In one prior art ballast circuit, a capacitor is connected in series with the secondary winding of the transformer for limiting the current after ignition. However, the capacitor increases the time required for reaching steady state illumination.
In the use of these prior art circuits a sinusoidal supply voltage is applied to the primary winding of the transformer; and thereby a sinusoidal voltage is also applied across the lamp. If a non-sinusoidal voltage having other than a perfect square-wave-type waveform containing no DC component were to be applied across the lamp in these prior art circuits, it would cause mercury ions within the lamp envelope to migrate toward one of the electrodes. As a result of mercury migration and predominant accumulation at one end of the lamp, the threshold voltage for lamp ignition becomes higher, the lamp becomes less efficient, and the brightness of steady state illumination also decreases.
It is desired to use low pressure gas discharge lamps, such as fluorescent lamps, in a changeable segmented alpha-numeric character display device, wherein a single lamp is used for each segment of each character. For such an application, it is required that the lamps be quickly ignited and be rapidly heated to a bright level of illumination, and that they have a uniform steady state illumination among the several segments of the display.
The prior art ballast circuit using loosely coupled transformers or series capacitors do not cause the lamps to operate at a bright level of illumination rapidly enough for satisfactory use as a changeable segmented display device.